Breast cancer is a leading cause of morbidity and mortality among women age 35 and older. Knowledge on the carcinogenic process is limited. Understanding the control of the proliferation of estrogen target cells is crucial to implement preventive, prognostic, diagnostic and therapeutic measures aimed at reducing the morbidity and mortality in these women. We adopted the premise that proliferation is a constitutive, dominant property of cells in metazoa. Thus, the centerpiece of our research is the identification of specific inhibitors of the proliferation of breast tumor cells. We identified human serum albumin (HSA) as such type of inhibitor. Using recombinant HSA we verified that the inhibitory effect was specific regarding the target cell type; among hormones and growth factors, only estrogens overcame this inhibitory effect. Moreover, no other protein was found to produce this inhibitory effect. Because human analbuminemic serum is also able to inhibit the proliferation of MCF7 cells, a paradox emerged. We found evidence consistent with the hypothesis that analbuminemic serum contains as truncated HSA polypeptide, and therefore, HSA may may be considered as a "prohormone". We will better to resolve this paradox and shed further light on the mechanism of action of HSA and estrogens on the proliferation of breast tumor cells during the period of support.